The invention relates to a vehicle tire, and in particular to a tire, the architecture of which is optimized to reinforce its endurance and its strength at high speed.
It is now conventional, in particular for tires for passenger vehicles intended to travel at high speeds, to use an additional ply of circumferentially oriented reinforcing threads to reinforce the crown. This ply may be arranged above the reinforcing crown plies at angles which are also conventionally used.
In such a configuration, this ply of circumferentially oriented reinforcing threads is the radially outermost ply of the crown of the tire and is the first to be subjected to the various attacks to which the tires may be subjected during travel.
Breaking of one or more of these circumferential reinforcing threads during travel may be harmful to the life of the tire, because such breaks reduce the hooping of the crown but also make it possible for water to infiltrate along the circumferential reinforcing threads, which may result in corrosion phenomena on the metal reinforcing threads.
Hereafter, xe2x80x9ccircumferential groovexe2x80x9d will be understood to mean a groove arranged on the radially outer surface of the tire and oriented in the circumferential direction or substantially in this direction and continuous over the entire circumference of the tire.
xe2x80x9cAxial width of a circumferential groovexe2x80x9d is understood to mean the maximum axial width of the groove when it is strictly circumferential or, when it is oriented substantially circumferentially, that is to say with slight angling or undulation relative to this direction, the axial distance between the two circumferential planes which are arranged on either side of the groove and are each tangent to one of the faces of the groove.
xe2x80x9cLinear densityxe2x80x9d is understood to mean the weight in grams of one thousand meters of a reinforcing thread. The linear density is expressed in tex. The stress to which a reinforcing thread is subjected or the modulus of this reinforcing thread are expressed in xe2x80x9ccN/texxe2x80x9d, cN meaning centinewton.
xe2x80x9cReinforcing threadxe2x80x9d is understood to mean any reinforcement element in the form of a thread which is able to reinforce a given matrix, for example a rubber matrix. As reinforcing threads, mention will be made, for example, of multifilament yarns, these yarns possibly being twisted on themselves or not, unit threads such as single cords of high elementary diameter, with or without a twist on themselves, cabled yarns or plied yarns (xe2x80x9ccordsxe2x80x9d) obtained by cabling or plying operations on these unit threads or these yarns, such reinforcing threads possibly being hybrid ones, that is to say, composite ones, comprising elements of different natures.
xe2x80x9cPlied yarnxe2x80x9d (or xe2x80x9cfolded yarnxe2x80x9d) is understood to mean a reinforcing thread formed of two single yarns or more assembled together by plying operations; these single yarns, which are generally formed of multifilament yarns, are first of all plied individually in one direction (S or Z direction of twist) during a first plying stage, then twisted together in the opposite direction (Z or S direction of twist, respectively) during a second plying stage.
xe2x80x9cAdherent reinforcing threadxe2x80x9d is understood to mean a reinforcing thread which has undergone an appropriate coating treatment, referred to as sizing or adherization treatment, capable of making this reinforcing thread, after suitable heat treatment, adhere to the matrix for which it is intended.
xe2x80x9cLaying pitch pxe2x80x9d of a reinforcing thread oriented substantially circumferentially and helically wound is understood to mean the transverse distance between the axes of two adjacent turns of reinforcing thread. The laying pitch is the inverse of the xe2x80x9claying density dxe2x80x9d, which corresponds to the number of turns of reinforcing thread contained in a given axial width. Conventionally, d is expressed in the number of reinforcing threads per decimeter (f/dm) and p in millimeters, so p=100/d.
The subject of the invention is a tire comprising a crown extended by two sidewalls and two beads, a carcass anchored in the two beads, in which the crown comprises radially from the inside towards the outside: at least one reinforcement ply formed of parallel reinforcing threads oriented at an angle xcex1 relative to the circumferential direction of between 10 and 75 degrees, at least one ply of reinforcing threads which are oriented substantially circumferentially and are helically wound and extend axially substantially above the assembly of said reinforcement ply, and a tread having tread patterns on its outer face with at least one circumferential groove.
This tire is characterized in that, in the zones arranged radially between the circumferential groove and the reinforcement ply formed of reinforcing threads oriented xcex1, the laying pitch for the reinforcing threads which are wound in a spiral is greater than or equal to the axial width of the circumferential groove.
Beneath the circumferential grooves of the tread pattern of the tire, the fact that the laying pitch of the reinforcing threads which are oriented circumferentially and helically wound is greater than or equal to the axial width of the groove means that, from one side of the groove to the other, there is at most only one winding of circumferential reinforcing thread. This embodiment therefore substantially reduces the probability of accidental breaking of these reinforcing threads and hence reinforces the strength of the tire overall.
This solution also has the advantage of avoiding excessive variation in width, when passing the grooves, of the belting rigidity provided by the circumferentially oriented reinforcing threads. This effect is all the greater, the higher the modulus of these circumferential reinforcing threads. This is also the case when the angle xcex1 of orientation of the reinforcing threads of the reinforcement plies increases, that is to say when the portion of the circumferential stresses which the circumferential reinforcing threads withstand increases relative to the portion which the reinforcement ply or plies having reinforcing threads oriented at the angle a relative to the circumferential direction withstand.
The circumferential reinforcing threads take up very high circumferential stresses. Consequently, any point of stoppage of these reinforcing threads is a zone of high shearing of the rubber mix which surrounds the reinforcing thread. Consequently, any point of stoppage is a zone favorable to the initiation and propagation of damage. The proposed solution has the advantage of not involving the occurrence of points of stoppage of the reinforcing threads beneath the grooves or in the vicinity thereof.
The proposed solution also has the advantage of not making the manufacture of the tire any more complex. It is a solution which is very easy to put into practice.
The laying pitch may advantageously be greater than or equal to twice the axial width of the circumferential groove. In this case, there is at least one half-circumference of the tire in which any axial section of the zone located between the groove and the reinforcement ply has no helically wound reinforcing thread.
Advantageously, the ply of reinforcing threads which are oriented substantially circumferentially is formed of at least two reinforcing threads helically wound simultaneously. This makes it possible to reduce the laying time for the ply. The maximum number of reinforcing threads which can be wound simultaneously is at most four.
Advantageously, the circumferential groove of the tire according to the invention has an axial width greater than or equal to 3.5 mm.
According to a specific embodiment, the tread of the tire is, over at least one given axial zone of the crown, in direct contact with the circumferentially oriented reinforcing threads. This facilitates building of the tire by reducing the number of products to be laid. When the tread comprises a first mix intended to come into contact with the ground, and an underlayer arranged radially beneath said first mix, it is the underlayer which is advantageously in direct contact with the circumferentially oriented reinforcing threads.
The reinforcing threads oriented substantially circumferentially and helically wound advantageously develop a stress under 3% deformation which is greater than 12 cN/tex, and preferably greater than 20 cN/tex. These reinforcing threads therefore have a high elasticity modulus at high deformations, which makes it possible for the ply which they form to assume all its functions, in particular the hooping of the crown at high speed.
These reinforcing threads may also have an initial modulus less than 900 cN/tex, and preferably less than 800 cN/tex. The low initial modulus of these reinforcing threads has the advantage of improving the comfort of the tire and reducing its coast-by noise at low speed.
Such reinforcing threads may be formed by the assembly of at least one single yarn of nylon associated with at least one aramid single yarn.